Execute mutation testing to evaluate test suite effectiveness. Use when performing specialized testing. Trigger with phrases like "run mutation tests", "test the tests", or "validate test effectiveness".
npxskills add jeremylongshore/claude-code-plugins-plus-skills--skill running-mutation-testsLoading…
Execute mutation testing to evaluate the effectiveness of a test suite by systematically introducing small code changes (mutants) and checking whether existing tests detect them. A killed mutant means the tests caught the change; a surviving mutant reveals a testing gap.
stryker.config.mjs with mutate patterns, test runner, and thresholds.setup.cfg or pyproject.toml with section.[mutmut]npx stryker run, mutmut run, or mvn pitest:mutationCoverage.| Error | Cause | Solution |
|---|---|---|
| Mutation run takes hours | Too many files in scope or slow test suite | Narrow mutate scope to critical modules; use --incremental mode; parallelize with --concurrency |
| All mutants survive | Tests only check for truthiness, not specific values | Strengthen assertions -- use toBe(42) instead of toBeTruthy(); add boundary checks |
| Equivalent mutant false positive | Mutation produces functionally identical code (e.g., x >= 0 vs x > -1) | Mark as equivalent in config; ignore in score calculation; document rationale |
| Out of memory during run | Too many concurrent mutation workers | Reduce --concurrency setting; increase Node.js --max-old-space-size; reduce shard size |
| Stryker "initial test run failed" | Test suite does not pass cleanly before mutations begin | Fix all failing tests first; ensure npm test exits 0; check test runner configuration |
Stryker configuration for TypeScript project:
// stryker.config.mjs
export default {
mutate: ['src/**/*.ts', '!src/**/*.d.ts', '!src/**/index.ts'],
testRunner: 'jest',
jest: { configFile: 'jest.config.ts' },
reporters: ['html', 'clear-text', 'progress'],
thresholds: { high: 80, low: 60, break: 50 },
concurrency: 4,
timeoutMS: 10000, # 10000: 10 seconds in ms
};
Example surviving mutant and fix:
Mutant: src/utils/discount.ts:15 -- ConditionalExpression
Original: if (total > 100)
Mutant: if (total >= 100)
Status: SURVIVED
Fix -- add boundary test:
it('does not apply discount at exactly 100', () => {
expect(calculateDiscount(100)).toBe(0);
});
it('applies discount above 100', () => {
expect(calculateDiscount(101)).toBe(10.1);
});
mutmut for Python:
# Run mutation testing
mutmut run --paths-to-mutate=src/ --tests-dir=tests/
# View surviving mutants
mutmut results
# Inspect a specific mutant
mutmut show 42
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Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).